This invention relates to an electrophotographic copying apparatus, and more particularly to an electrophotographic copying apparatus which permits the making of a plurality of copies from one latent electrostatic image.
In general, the electrophotographic copying method employing a visible image transfer, is known using a series of steps consisting of the sensitizing of a photoconductor by electrical charging, the exposing of the photoconductor to form a latent electrostatic image, the developing of the latent image with a developer, the transferring of the developed visible image to a recording sheet or other materials, the electrical quenching of the photoconductor, and the cleaning of the photoconductor. Usually, this process is performed by rotating a drum-shaped or belt-shaped photoconductor so as to move the peripheral surface of the photoconductor in one direction.
In this specification, the electrophotographic copying method of the above-mentioned type is referred to as the copying method by single-copy process.
The latent electrostatic image formed on the photoconductor is stable for a comparatively long period of time in the dark. A copying method utilizing this characteristic of the latent image is known, which permits obtaining of copies from the same latent image by repeating both development of the latent image and the transferring of the developed image, once the latent image is formed. The copying method of this type is referred to as the copying method of multicopy process in this specification.
Since it is possible to adopt selectively both the copying method by single-copy process and the copying method by multi-copy process in the same copying apparatus by changing the control system of the copying apparatus, an electrophotographic copying machine, which permits selection of both the copying methods, has already been known.
The advantage of the copying method by multi-copy process is that since a plurality of the same copies can be obtained by a single exposure, and accordingly, since the number of cleanings can be reduced, the power consumption can be reduced. Furthermore, since from the second copy on, development and image transfer can be performed by use of the already formed same latent image, the copying time can be shortened significantly.
Naturally, an unlimited number of copies cannot be obtained from the same latent electrostatic image. With the increase in the number of repeated uses of the latent image, the image quality obtained is lowered. At the present technique level, approximately 30 copies from the same latent electrostatic image are practically usable in the case of line copying, and in the case of large, solid-image area copying, approximately 15 copies are practically usable.
In the case of the line copy work, for example, when 50 copies are required from the same original, they can be obtained in the following manner using the copying method by multi-copy process. First, the counter for use in multicopy is set for the maximum 30 copies, and initially 30 copies are made. Then the above-mentioned counter is set for 20 copies, and the remaining 20 copies are made.
However, in the case where all of these copies are to be used commercially, all the copies do not have a sufficient copy quality to be acceptable as a commercial product.
Out of 30 copies obtained by the multi-copy process from the same electrostatic image, if 10 copies have a sufficient image quality to qualify as a product to be commercialized, and 50 copies having such image quality are required from the same original, the multi-copy process has to be repeated five times by setting the copy counter for 10 copies each time when the conventional copying apparatus is utilized.
Furthermore, as shown in FIG. 4, when 10 l copies can be obtained from the same latent image, but 12 copies from the same original are required, the latent image is formed at the first copy and at the tenth copy. However, the electric potential of a latent image is decreased in proportion to the number of repeated uses of the latent image. Therefore, the image density and resolution of the teeth copy is significantly lower than those of the first copy and the eleventh copy since with the increase of the number of repeated uses of the latent electrostatic image, the image quality of the copies is successively lowered. Also, the quality of the copies becomes uneven.
Furthermore, when the original is replaced with a subsequent original after the formation of the last latent image, the smaller in number the copies to be made after the final formation of the same latent image, the less the time allowed for replacing the subsequent original before the end of copying of the prior original, so the more the total copying time, causing a waste of time.